The present invention relates to a defect inspection method and a defect inspection apparatus that uses this, and more specifically, to a defect inspection method for detecting defects of a minute pattern formed on a substrate through a film process that is typified by a semiconductor manufacture process and a manufacture process of flat panel display, foreign materials, etc. and a defect inspection apparatus that uses this.
As a conventional apparatus for detecting defects of a specimen, there is a patent of International Publication WO 2003/083560. This inspection apparatus is equipped with a dark field detecting optical system that detects scattered light on a wafer by illuminating the wafer surface slantingly with a light of a single wavelength. Diffracted light from a periodic pattern coming into this optical system is shielded by a spatial filter disposed at a rear side focal point position (an exit pupil position). As this spatial filter, a configuration that uses a liquid crystal filter conformed to ultraviolet rays is shown.
Various patterns are formed on the semiconductor wafer and kinds of defects have also great varieties according to generation causes. In the optical dark-field detection system, a laser is used as a light source, which provides an illumination light of a single wavelength.
However, with the light of the single wavelength, there is the case where the scattered light can hardly be obtained from the defect depending on optical constants of a defective material and the shape and structure around the defect. As one example, although a silicon oxide film is used as an electric insulation film of the patterns that are stacked, an optically transparent oxide film generates thin film interference; therefore the intensity of the scattered light will vary depending on its film thickness. Because of this phenomenon, the scattered light becomes extremely small with a film thickness condition of weakening light rays and becomes undetectable. As a countermeasure against this, the number of illumination wavelengths is increased to a plural number. By this modification, even under a film thickness condition under which the amount of detected light is insufficient with a single wavelength, a probability of being able to secure the amount of light necessary for defect detection at the other wavelength(s) is increased.
In this way, although it is possible to improve a capture ratio of defects with illumination of a plurality of wavelengths, the diffracted light from the periodic pattern, such as the memory cell part, has a different position of a diffracted image for each wavelength because a diffraction angle is expressed by a function of wavelength. FIG. 3 shows a schematic diagram of the diffracted image when the periodic pattern is illuminated by the illumination lights of wavelengths λ1, λ2 (λ1 is a relatively short wavelength) (hereinafter referred to as a λ1 light and a λ2 light). This diagram is a schematic diagram of the diffracted image formed at an exit pupil 400 of an objective lens. FIG. 3A shows a diffracted image (diffracted light) 410 of the λ1 light, and FIG. 3B shows a diffracted image (diffracted light) 430 of the λ2 light. The diffracted image 410 occurs in a direction of periodicity of the pattern. An example of FIGS. 3A, 3B, and 3C is an example of the diffracted image of a pattern that is formed periodically in two directions intersecting at right angles.
As means for shielding these diffracted images, there is a technique of shielding the image by fitting a light shielding belt 420 to a pitch of the diffracted image. When lights of two wavelengths λ1, λ2, are simultaneously cast for illumination, regarding the diffracted images actually detected, the two diffracted images of wavelength λ1, λ2, are detected at the exit pupil 400 of the objective lens. Because of this, the number of the diffracted images becomes large as shown in FIG. 3C. If these images are intended to be shielded, the number of the light shielding belts will become large and an aperture ratio of the exit pupil will lower. Since this leads to lowering of substantial resolution, there is a problem to be solved that contrast of a minute defect decreases and defect detection sensitivity lowers.
In addition, when the liquid crystal filter is used as the spatial filter, it is necessary that the scattered light is filtered so as to become a linearly polarized light and alignment of the liquid crystal is electrically controlled to make it perform optical rotation. It becomes possible to control the transmittance of the light that is transmitted through a polarizing plate disposed on an image plane side depending on the amount of this optical rotation.
However, the polarization state of the scattered light changes according to a shape, a structure, a material, etc. of the pattern and the defect. Therefore, if the scattered is filtered so as to become a linearly polarized light one the object side (wafer side) of the liquid crystal, in the case where the defect scattered light is polarized in a direction perpendicular to a filter transmission axis, the scattered light of the defect will be shielded and accordingly it will become impossible to detect the defect.